With recent development of portable devices, there has been a strong demand for higher-capacity secondary batteries for use in such devices. For example, a nickel hydroxide powder for a positive electrode material of alkaline secondary battery has been improved by forming a solid solution with cobalt to improve the utilization ratio of an alkaline secondary battery at high temperatures or by forming a solid solution with zinc or magnesium to improve the lifetime characteristics of an alkaline secondary battery.
Alkaline secondary batteries have come to be used as high-output power sources such as power sources for hybrid cars; therefore, there has been a strong demand not only for such improvement in utilization ratio at high temperatures or lifetime characteristics but also for improvement in output characteristics. However, a nickel hydroxide powder for a positive electrode active material of alkaline secondary battery is an electrical insulating material and poor in conductivity, which causes a problem that an electrical current does not sufficiently run through nickel hydroxide; therefore, the electrochemical availability of nickel hydroxide is low.
In order to solve such a problem, a cobalt compound such as cobalt oxide or cobalt hydroxide is added as a conductive material to ensure conductivity between nickel hydroxide particles. Such a cobalt compound added is dissolved in a high-concentration alkali metal hydroxide solution used as an electrolyte in an alkaline secondary battery, and is oxidized and deposited as cobalt oxyhydroxide on the surface of nickel hydroxide particles during electrical charge so that electrical conductivity is developed and a conductive network is formed between the nickel hydroxide particles.
A positive electrode of a nickel hydroxide powder having a cobalt compound added thereto is generally produced by the steps of mixing a nickel hydroxide powder, a cobalt compound powder, and a hinder to form a paste, filling the pores of a three-dimensional metal porous body, such as a foamed metal (made of nickel metal), with the paste, and subjecting the three-dimensional metal porous body to drying and pressing. However, the cobalt compound powder mixed together with the binder is not necessarily well dispersed in the nickel hydroxide powder. Therefore, the positive electrode has a problem that its utilization ratio is significantly reduced under the conditions of use during high-load electrical charge.
As a means for solving such a problem, a method has been proposed in which the surface of particles of a nickel hydroxide powder is coated with a cobalt compound. For example, Patent Literature 1 proposes a nickel active material for storage battery mainly comprising nickel hydroxide particles having a β-type cobalt hydroxide thin layer formed thereon. Patent Literature 1 states that this nickel active material is obtained by depositing a nickel hydroxide powder from a nickel salt in an aqueous alkali solution, immersing the nickel hydroxide powder in an aqueous solution of cobalt sulfate or cobalt nitrate, and neutralizing the aqueous solution with an aqueous alkali solution.
Further, Patent Literature 2 proposes a method for producing a nickel hydroxide powder coated with cobalt hydroxide, in which a cobalt-containing aqueous solution and an ammonium ion supplier are simultaneously, continuously, and quantitatively supplied to a nickel hydroxide powder-containing aqueous solution adjusted to pH 11 to 13 with a caustic alkali.
Further, Patent Literature 3 proposes a method in which a cobalt ion-containing aqueous solution, with a supply rate of 0.7 g/min or less in terms of cobalt per kilogram of the raw nickel hydroxide powder, and an ammonium ion-containing aqueous solution are supplied to a suspension of a raw nickel hydroxide powder to achieve a nickel ion concentration of 10 to 50 mg/l and a cobalt ion concentration of 5 to 40 mg/l while the pH, temperature, and ammonium ion concentration of the suspension are kept at predetermined values.